This invention pertains to field emitting devices and, in particular, to field emitting devices comprising field-concentrating nanoconductor assemblies and to methods for making such devices.
Field emitting devices are useful in a wide variety of applications. A typical field emitting device comprises a field emitting assembly composed of a cathode and a plurality of field emitter tips. The device also typically includes a grid closely spaced to the emitter tips and an anode spaced further from the cathode. Voltage induces emission of electrons from the tips, through the grid, toward the anode. Applications include flat panel displays, klystrons and traveling wave tubes, ion guns, electron beam lithography, high energy accelerators, free electron lasers, and electron microscopes and microprobes. One of the most promising applications is thin, matrix-addressed flat panel displays. See, for example, Semiconductor International, December 1991, p.46; C. A. Spindt et al., IEEE Transactions on Electron Devices, vol. 38, pp. 2355 (1991); I. Brodie and C. A. Spindt, Advances in Electronics and Electron Physics, edited by P. W. Hawkes, vol. 83, pp. 1 (1992); and J. A. Costellano, Handbook of Display Technology, Academic Press, New York, pp. 254 (1992), all of which are incorporated herein by reference.
A conventional field emission flat panel display comprises a flat vacuum cell having a matrix array of microscopic field emitters formed on a cathode and a phosphor coated anode disposed on a transparent front plate. An open grid (or gate) is disposed between cathode and anode. The cathodes and gates are typically intersecting strips (usually perpendicular) whose intersections define pixels for the display. A given pixel is activated by applying voltage between the cathode conductor strip and the gate conductor. A more positive voltage is applied to the anode in order to impart a relatively high energy (400-5,000 eV) to the emitted electrons. For additional details see, for example, the U.S. Pat. Nos. 4,940,916; 5,129,850; 5,138,237 and 5,283,500, each of which is incorporated herein by reference.
A variety of characteristics are advantageous for field emitting assemblies. The emission current is advantageously voltage controllable, with driver voltages in a range obtainable from xe2x80x9coff the shelfxe2x80x9d integrated circuits. For typical CMOS circuitry and typical display device dimensions (e.g. 1 xcexcm gate-to-cathode spacing), a cathode that emits at fields of 25 V/xcexcm or less is generally desirable. The emitting current density is advantageously in the range of 1-10 mA/cm2 for flat panel display applications and  greater than 100 mA/cm2 for microwave power amplifier applications. The emission characteristics are advantageously reproducible from one source to another and advantageously stable over a long period of time (tens of thousands of hours). The emission fluctuations (noise) are advantageously small enough to avoid limiting device performance. The cathode should be resistant to unwanted occurrences in the vacuum environment, such as ion bombardment, chemical reaction with residual gases, temperature extremes, and arcing. Finally, the cathode manufacturing is advantageously inexpensive, e.g. devoid of highly critical processes and adaptable to a wide variety of applications.
Previous cathode materials are typically metal (such as Mo) or semiconductor (such as Si) with sharp tips. While useful emission characteristics have been demonstrated for these materials, the control voltage required for emission is relatively high (around 100 V) because of their high work functions. The high control voltage increases damage due to ion bombardment and surface diffusion on the emitter tips and necessitates high power densities to produce the required emission current density. The fabrication of uniform sharp tips is difficult, tedious and expensive, especially over a large area. In addition, these materials are vulnerable to deterioration in a real device operating environment involving ion bombardment, chemically active species and temperature extremes.
Diamond emitters and related emission devices are disclosed, for example, in U.S. Pat. Nos. 5,129,850, 5,138,237, 5,616,368, 5,623,180, 5,637,950 and 5,648,699 and in Okano et al., Appl. Phys. Lett. vol. 64, p. 2742 (1994), Kumar et al., Solid State Technol. vol. 38, p. 71 (1995), and Geis et al., J. Vac. Sci. Technol. vol. B14, p. 2060 (1996), all of which are incorporated herein by reference. While diamond field emitters have negative or low electron affinity, the technology has been hindered by emission non-uniformity, vulnerability to surface contamination, and a tendency toward graphitization at high emission currents ( greater than 30 mA/cm2).
Nanoscale conductors (xe2x80x9cnanoconductorsxe2x80x9d) have recently emerged as potentially useful electron field emitters. Nanoconductors are tiny conductive nanotubes (hollow) or nanowires (solid) with a size scale of the order of 1.0-100 nm in diameter and 0.5-10 xcexcm in length. Carbon nanotubes, which are representative, are a stable form of carbon which features high aspect ratios ( greater than 1,000) and small tip radii of curvature (1-50 nm). These geometric characteristics, coupled with the high mechanical strength and chemical stability, make carbon nanotubes especially attractive electron field emitters. Carbon nanotube emitters are disclosed, for example, by T. Keesmann in German patent No. 4,405,768, and in Rinzler et al., Science, vol. 269, p.1550 (1995), De Heer et al., Science, vol. 270, p. 1179 (1995), Saito et al., Jpn. J. Appl. Phys. Vol. 37, p. L346 (1998), Wang et al., Appl. Phys. Lett., vol. 70, p. 3308, (1997), Saito et al., Jpn. J. Appl. Phys. Vol. 36, p. L1340 (1997), Wang et al., Appl. Phys. Lett. vol. 72, p 2912 (1998), and Bonard et al., Appl. Phys. Lett., vol. 73, p. 918 (1998), all of which are incorporated herein by reference. The synthesis of conductive nanowires based on semiconductor materials such as Si or Ge has also been reported. See, for example, A. M. Morales et al. Science, Vol. 279, p. 208 (1998), which is incorporated herein by reference.
Nanoconductors are which are grown in the form of randomly oriented, needle-like or spaghetti-like powders that are not easily or conveniently incorporated into a field emitter device. Due to this random configuration, the electron emission properties are not fully utilized or optimized. Many nanoconductor tips may be buried in the mass. Ways to grow nanoconductors in an oriented fashion on a substrate are disclosed in Ren et al., Science, Vol. 282, p. 1105 and Fan et al., Science, Vol. 283, p. 512, both of which are incorporated herein by reference.
This invention is predicated on applicants"" discovery that a highly oriented nanoconductor structure alone does not guarantee efficient field emission. To the contrary, the conventional densely populated, highly oriented structures actually yield relatively poor field emission characteristics. Applicants have determined that the individual nanoconductors in conventional assemblies are so closely spaced that they shield each other from effective field concentration at the ends, thus diminishing the driving force for efficient electron emission.
In accordance with the invention, an improved field emitting nanoconductors assembly (a xe2x80x9clow density nanoconductor assemblyxe2x80x9d) comprises an array of nanoconductors which are highly aligned but spaced from each other an average distance of at least 10% of the average height of the nanoconductors and preferably 50% of the average height. In this way, the field strength at the ends will be at least 50% of the maximal field concentration possible. Several ways of making the optimally low density assemblies are described along with several devices employing the assemblies.